Skip IP::isInRange() test if input is not in CIDR notation

IP:isInRange() performs several moderately expensive tests via regular
expression matching that can cause unwanted load with a sufficiently
complicated runtime configuration. Check the $wgSquidServersNoPurge
value to see if it includes the CIDR mask separator token ('/') before
continuing on to perform those more expensive tests.

Follows up Ia81bed7d8b42a2d2b200a3ce45a74e3784cdca2a

Bug: 52829

Change-Id: Ia557cb54f3a1bf67c8282140ab748369faa83028